The present invention relates to the field of voltage multiplier circuits, and more particularly, to a voltage multiplier incorporating diodes fabricated by bonding silicon to an insulating substrate and a method of fabricating same.
Voltage multiplier circuits employing capacitors and diodes to effect voltage multiplication by means of charge pumping are well known. Applications of voltage multiplier circuits include providing relatively high voltage within integrated circuits, and multiplying the voltage output of high voltage power supplies.
The simplest method of producing a voltage multiplier circuit is to wire together discrete diodes and capacitors. This method is generally employed for high voltage power supply applications. Another method of producing a voltage multiplier circuit is to integrate the diodes and capacitors on a semiconducting substrate as is done for applications within an integrated circuit. Both of these approaches have disadvantages for some applications. In the case of the discrete element circuits, the size of the circuit elements limits the number of multiplier stages which can be practicably implemented. Also, the frequency of the oscillatory charge pumping voltages is limited by the large size of the circuit elements. In the case of the integrated voltage multiplier circuits, the voltages which can be attained are limited by voltage induced breakdown between circuit elements and the semiconducting substrate on which the circuits are fabricated. The semiconducting substrate can also cause capacitive shunting of the oscillatory pumping voltages at high frequency.
An approach which avoids the limitations of prior art discussed above is to use an integrated voltage multiplier circuit fabricated on an insulating substrate. One silicon-on-insulator technology which can be employed is epitaxially grown silicon-on-sapphire (SOS). The problem with epitaxially grown SOS is that the high temperatures required in the fabrication process and differential thermal expansion effects cause relatively high reverse current leakage in SOS diodes. Low reverse leakage current is necessary for efficient voltage multiplier circuit operation. Thus, a need exists for voltage multiplier circuits having high quality diode devices integrated on an insulating substrate and a method for fabricating such circuits.